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Discussion |
1 1 British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK (e-mail: Douglas.Peacock{at}virgin.net)
2 2 School of Environmental Science, University of Ulster, Coleraine BT52 1SA (e-mail: m.mccabe{at}ulster.ac.uk)
3 3 Department of Geosciences, Oregon State University, Corvallis, OR 97331, USA
4 4 Oxford University Centre for the Environment, South Parks Road, Oxford OX1 3QY, UK
J. D. Peacock, M. Armstrong, M. A. E. Browne, N. R. Golledge & M. S. Stoker write: We are unable to support the view of McCabe et al. (2007) that there were significant readvances of the Scottish Ice-sheet at Lunan Bay and near Perth. The authors provide no criteria for readvances of glaciers such as terminal moraines, a second till and/or deforming bed, and rafting and overriding of marine sediments. However, we believe that the four new 14C dates in their table 1 are useful additions to knowledge.
The supposed Perth Readvance in the type area. We agree that the till at the base of the Almondbank was produced by advancing ice. However, because such till is widespread in eastern Scotland, its occurrence does not of itself support the concept of a readvance, and none of the criteria noted above are satisfied. Indeed, the only indication of a possible very local readvance in the Perth area is near Moneydie, where a section formerly exposed two tills and highly deformed lacustrine silts (Paterson 1974). This we could attribute to the expected readjustment of ice-streams during retreat. Menzies & Van Der Meer (1998) concluded that the evidence here did not support a major readvance, but rather represented ‘a single winter event’. The package of sediments was described as ‘part of a tectonised, sub-glacial, deforming bed sequence’.
In the upper Forth Estuary, near Grangemouth, Sissons & Smith (1965) showed that the Main Perth Shoreline is associated with the eastern limit of a stillstand or readvance of ice, which they correlated with the ‘Perth Readvance’. However, in the Tay–Earn area, the marine limit above the Main Perth Shoreline can be followed almost to Crieff, some 25 km west of Perth, where there are fluvial terraces relating to a sea level at or near that of the Main Perth Shoreline (Browne 1980). Thus the limit of a readvance or stillstand contemporaneous with that near Grangemouth is not part of a ‘Perth Readvance’, but needs to be looked for to the west of Crieff (Peacock 2003).
The supposed Lunan Bay Readvance. Golledge & Stoker (2006) showed conclusively that the last glacier ice in Strathmore streamed to the NE along the whole length of the valley, as well as offshore, and was not crossed by later ice emanating from the Highlands. The only detailed section examined by Rice (1960, p. 249) shows sheet gravel unconformably overlying undisturbed marine, probably deltaic sediments. None of the criteria listed above have been seen. The minor folds and contortions observed by Rice in the marine strata at two localities are likely to be penecontemporaneous slump folding, convolute bedding, or disturbances consequent on the melting of buried ice. We conclude that the glacial morphology in the Lunan and Brothock valleys, which is similar to that at Almondbank, resulted from the melting of buried ice blocks as ice retreated inland. The concept of a late major readvance extending SE from the Highlands, as advocated by McCabe et al. (2007), is therefore mistaken.
Radiocarbon ages. The new 14C ages from Lunan Bay (c. 17.0 and 17.7 ka bp, adjusted for a reservoir age of 400 years), support the diachronous nature of the Errol Clay Formation, but do not of themselves support a Lunan Bay Readvance. Of the already published dates from Gallowflat, the two most reliable (c. 13.8 ka bp), are from Division B of the local threefold Errol Clay Formation (Peacock 2003), whereas the new dates of c. 13.7 ka bp are from the underlying Division A. Although slightly out of order, their similarity confirms the probability of rapid deposition. However, the date of c. 12.1 ka bp from Dryleys (McCabe et al. 2007, fig. 1) relates to the Windermere Interstadial and, if from the Errol Clay Formation, would be expected to be much greater than 13 ka bp (Peacock 1999), and similar to those from Lunan Bay (above). For the St. Fergus Silts, the dates of c. 14.3 and c. 14.9 14C ka bp (Peacock 2003) are from a shell bed 4.5 m below ground level, and not from the section illustrated by McCabe et al. (2007, fig. 1). Peacock advised that the significance of these dates should not be overemphasized because the shells remaining in the collection are chalky. That caution is now reinforced by the dates from Lunan Bay (above). Together with a geographical situation far from the centre of isostatic uplift, they suggest that the St. Fergus Silts are likely to be many thousand years older than previously considered.
Conclusions. Although we subscribe to the view that there were readvances of the Scottish Ice-sheet during the latter part of the Dimlington Stadial, the criteria for such readvances are lacking both near Perth and from the Lunan Bay and Brothock valleys. In contrast, the glaciers of the later Loch Lomond Readvance are defined by significant terminal moraines, and include transported and deformed marine clay (Simpson 1933; Peacock 1992). Taken together with the other evidence cited above, we are unable to support the concept of either a readvance near Perth, or a (probably earlier) Lunan Bay Readvance.
23 March 2007
A. M. McCabe, P. U. Clark, D. E. Smith & P. Dunlop reply: We interpreted two ice-sheet readvances in eastern Scotland during the last deglaciation based on stratigraphic evidence from exposures, with age constraints for the readvances based on new accelerator mass spectrometry 14C radiocarbon dates. Peacock et al. reject our interpretation because we do not provide criteria that, according to them, are required to identify a readvance. We agree with these criteria, and in fact our interpretations are based on one of those listed by Peacock et al.; namely, evidence of overriding of marine sediments. We are thus at a loss to explain why Peacock et al. overlook this point, which is clearly documented in our original paper. For their benefit, we thus reiterate the evidence. At Lunan Bay, we dated monospecific forams derived from marine muds that are unconformably overlain by gravel that was deposited in contact with ice, as shown by the kettled topography. A similar stratigraphy exists at Bertha Park, where thick beds of marine mud are overlain by coarse-grained gravel, forming kettled ice-contact topography. Clearly these stratigraphies require ice to readvance over the open-water marine muds at both sites to produce the kettled topography and to explain the change in the facies succession (open-water muds with little ice rafting to ice-contact deposition with glacial ice). Peacock et al. quote Paterson (1974) and state that the sequences ‘could be attributed to the expected readjustment of ice-streams during ice retreat’. The precise meaning or the inferred processes here are at best unclear, as is the inference on the unsupported age duration (a single winter event) of the related sediment deformation at Moneydie.
Peacock et al. refer to Golledge & Stoker (2006) in purporting to show ‘conclusively’ that the last glacier ice in Strathmore streamed to the NE along the whole length of the valley, thus conflicting with a Lunan Bay Readvance. However, their event is an earlier event and in any case is based on undated ice flow indicators, which may relate to different ice-sheet events. The ice-contact deposits with numerous, large kettle holes overlying muds in the Lunan and Brothock valleys must have consisted of ice flow from Strathmore because of erratics derived from the eastern Grampians (Synge 1956; Rice 1960).
Our suites of dates come from raised marine muds demonstrating that marine deposition in eastern Scotland started before 17.7 14C ka bp and continued after 12.1 14C ka bp. Peacock et al. suggest that the diachronous nature of the Errol Clay Formation covers much of this time interval. However, we have identified significant hiatuses from the succession at Bertha Park and Lunan Bay and therefore now question the usefulness of the term Errol Clay Formation. If this term is continued, we believe it tends to mask important events in deglacial history, reinforcing the misconception of monotonic ice retreat with minor stillstands.
Peacock et al. agree that the Main Perth Shoreline is closely related to a stillstand or readvance of the ice margin at Grangemouth in the Forth valley. They then argue that there should be a similar ice-sheet limit somewhere near Crief, some 25 km west of Perth. However, this proposal is by no means clear because age-constrained field evidence from marine muds, data to support the supposed marine limit, and assessments of marine microfaunas from marine muds have not been presented. Although we agree that this area needs remapping, we cannot see why the lack of reliable field evidence from this area should invalidate the firm stratigraphic evidence for the ice limit and terminal gravel outwash at Bertha Park (Perth) in the Tay valley.
The Main Perth Shoreline is central to the concept of the Perth Readvance, for its terraces continue down-valley from kettled outwash terraces often without interruption, constituting an almost seamless transition from glacial limit to contemporary shoreline. The shoreline terraces, often over 100 m wide, and composed of fining-downwards silts and sands (often laminated at the base, as could be seen in the recent excavations for the road system at Kincardine Bridge on the Firth of Forth), can be followed several tens of kilometres down-valley from the ice limits, demonstrating the availability of sediment from readvancing ice in the valleys. They are less evident along the open coastline to the east, however, because of a lesser sediment supply there. The drop of 15 m in the marine limit inland of the second Perth shoreline at Stirling, originally contested by Browne et al. (1981a, b), and later reasserted (Smith & Cullingford 1981; Smith 1997), is substantial, given that the highest shoreline there reaches 38 m. The shoreline terraces are most conspicuous in the Forth and Tay valleys (e.g. Cullingford 1977), standing out from both earlier and later terraces. These patterns are best explained by a significant ice-sheet readvance.
The Main Perth Shoreline, which is the highest (and most extensive) shoreline of the Perth sequence, was identified from inclusive morphological mapping at a scale of 1:10 000 or 1:10 560 and its altitude measured along the inner margin, or highest point, of terraces at 50 m or 80 m intervals by instrumental levelling, yielding several hundred altitudes, which allowed the shoreline gradient to be defined first for each of the Forth and Tay valleys by linear regression and later spatially across SE Scotland by trend surface analysis (Smith et al. 1969), both to a high degree of confidence. Comparison between the gradient of the Main Perth Shoreline and the gradients of earlier shorelines identified by the same technique in East Fife showed that the gradients of the earlier shorelines sloped increasingly less steeply as they became younger until the lowest shoreline that could be recognized sloped only very slightly more steeply than the Main Perth Shoreline. However, in areas where both shorelines exist in a 25 m vertical sequence, the Main Perth Shoreline is at least 12 m lower (illustrated by Smith 1997). Such a relationship, in which a sequence of shorelines progressively declining in gradient is interrupted by two parallel shorelines, implies that glacio-isostatic uplift had at least been retarded at the time of the Main Perth Shoreline, implying an increase in ice load during the Perth Readvance.
27 June 2007
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References |
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